This invention relates to the preparation of fiber-reinforced thermosettable resin articles. In a specific embodiment, the invention relates to a process and apparatus for impregnation of a glass substrate with a solventless thermosettable resin system.
The manufacture of the cured thermosettable resin base of an electronic circuit board begins with the impregnation of a fibrous glass substrate with a liquid thermosettable resin system. The resin-impregnated substrate is then partially cured to form a "prepreg." A set of layered prepregs is then heated under pressure to fully cure the resin and to form a hard laminate, which serves as the base for electric circuitry.
Although there exist thermosettable resins, such as low molecular weight epoxy resins, which are liquid at room temperature, current circuit board requirements make it necessary to use high-performance resins systems which are solids or viscous liquids at room temperature and to apply the resins to the substrate in melt or solution form. Attempts to process thermosettable resins in the melt, however, have not been successful because of the difficulty of achieving good "wet-out," or saturation of the fiber by the resin, and also because the high temperatures necessary to melt the resin cause the resin to cure prematurely, further adding to the wet-out problem.
Current commercial processes for preparing prepregs apply the resin to the substrate using an organic solution of the resin. Solution processes must include a step, usually carried out in conjunction with partial curing of the resin, in which the solvent is removed from the prepreg by heating the solvent to its volatilization temperature. Such a process has a number of disadvantages: First, it requires the disposal or discharge of the organic volatiles. Second, volatilization of the solvent from the uncured resin can result in the presence of voids and irregularities in the prepreg and in the cured laminate. Furthermore, a considerable amount of time is required for the solvent removal step. A method for applying resin to the substrate which did not require solvents would thus have environmental, quality and efficiency advantages.
Processes for applying liquid-form resins to the substrate include passing the substrate through a resin bath, as illustrated in U.S. Pat. No. 4,767,643, and coating a non-porous release sheet with liquid resin and then pressing the release film against the porous substrate to transfer the resin thereto, as described in U.S. Pat. No. 4,139,591. The former technique suffers from problems associated with the tendency of resin in the reservoir to "advance," or partially cure, if it is not immediately taken up by the substrate, and the latter method suffers from the inconvenience and expense of processing the release sheet. It would be desirable to develop techniques for resins application which do not involve the use of a resin bath or a release sheet.
It is therefore an object of the invention to provide a process and apparatus for impregnating a fibrous substrate with a thermosettable resin. In a specific aspect, it is an object of the invention to provide a process and apparatus for impregnation of a fibrous glass substrate with a solventless thermosettable resin system.